There are a number of polymer resist systems for photolithography. The most common system is a single layer of a sensitive resist material coated on a substrate which is exposed to patterned radiation (e.g., electromagnetic radiation such as light, E-beam, X-ray), developed in a solvent and then used in a further process, for example as an etchant barrier. This traditional system, which can be either positive or negative acting, has many shortcomings in the low micron and sub-micron image definition range. A major problem faced is achieving fine resolution along with line-width control, particularly on reflective substrates. Resolution is limited by resist contrast as well as optical imaging resolution. Poor resist contrast results from linear effects of light intensity on the photochemical process in the resist material. Another problem is that the developer acts isotropically and produces sloped sides and poor reproduction of the mask pattern (e.g., undercutting or grading). Developing solvents are expensive, often toxic, and introduce defects.
Multi-layered resist systems have been designed to overcome many of the single layer process problems, and although good line-width control is achieved, these systems are highly complex and not amenable to a wide variety of production settings.
Single-layered dry process resist systems are available such as described in U.S. Pat. Nos. 4,307,178 and 4,389,482 which use plasma etching and reactive ion etching to develop the resist. These systems have poor contrast and allow loss of resist during development.
"Self developing" photoresist systems are single layer polymers which photo-decompose, volatilize and vaporize when exposed to U.V. light. U.S. Pat. No.4,588,801 discloses the use of polysilanes in this type of system. The use of nitrocellulose as a self-developing resist is discussed in Self-developing Resist With Submicrometer Resolution and Processing Stability, Gies et al, Appl. Phys. Letts., 43, p. 74, July 1983 and describes that high aspect ratio lines are achievable. These systems have high photochemical quantum yields and are capable of high resolution, but require high pulse power and high U.V. exposure. They also create debris which is left on the surface.
U.S. Pat. No. 4,458,994 teaches the use of an excimer laser as the radiation source in optical lithography, giving finer pattern resolution in short exposure times using conventional resists.
U.S. Pat. No. 4,414,059 teaches the use of an excimer laser in a one step process to pattern a layer of resist by ablative photodecomposition using U.V. radiation at power densities sufficient to cause fragmentation of the resist polymer chains.
EP 248,779 is an application for a single layer dry development positive resist system. It teaches coating a substrate with a polymer admixed with a photosensitive cross-linking agent, exposing the coated substrate through a mask to U.V. light, treating the layer with a silicon compound and plasma etching to remove the irradiated areas to form the desired positive pattern.
EP 184,567 is an application for the recently announced "Desire" process which is a single layer dry development negative resist system. The coated polymer resist, when imaged with standard U.V. light, chemically reacts with silicon containing compounds where exposed. The imaged polymer is then developed with an oxygen plasma. The silylated areas form protective oxides on the surface that stop further etching, thus leaving these areas to form an image. This system is severely limited by the resist material that can be used because of the nature of the U.V. exposure changing the chemical activity of the resist so it selectively absorbs silicon compounds which then become an oxygen plasma barrier during the developing stage.
U.S. Pat. Nos. 4,868,006, 4,879,176 and 4,822,451 disclose the formation of quasi-amorphous regions on semicrystalline polymer surfaces by pulsed energy treatment. A positive resist film is shown in one example of the last patent.